This invention relates to titanium alloys and has particular reference to titanium alloys intended for use at high temperatures.
In aerospace applications, particularly in aero-engines, the need to develop more efficient engines leads inevitably to a need to develop alloys capable of withstanding high temperatures in use. In many cases the alloys are developed specifically for their high creep strength at elevated temperatures. In some cases, however, the levels of creep strength are somewhat less important than the absolute levels of strength which the alloy can withstand. It will be appreciated that creep comprises extension of the alloy under load over long periods of time at elevated temperatures. Strength is more concerned with the ability of the alloy to withstand high loads not necessarily imposed for particularly long periods of time.
The present invention is concerned with the development of an alloy which has a fine and good balance of properties including resistance to crack propagation, high strength at elevated temperatures, weldability, a reasonable density and reasonable ductility. The alloys are frequently used in large sections. In such sections the ability of the material to be used in the welded condition is such that it enables engines to be designed to be assembled by welding.
By "weldable" as is used herein is meant that the material can be used commercially in the welded condition and has a micro-structure such that it can be used in the welded condition. The term weldable is not intended when used herein merely to mean that two pieces of the alloy can be joined together by welding.
Because the alloys of the present invention are frequently used in large sections it is important that the matter of depth hardenability be considered. The alloys of the present invention, as is common with most, if not all, titanium alloys, are used in the heat treated condition. The alloys are not used in the as-cast condition. Clearly if the alloy is heat treated by a process which involves quenching or fast cooling it is important that the properties of the alloy should be reasonably constant throughout the section.
The present invention is concerned with alloys which are heat treatable such that the properties are relatively independent of the thickness of the section treated and the present invention is, in part, based on the unexpected discovery that certain compositions may be heat treated by quenching without giving large variations in all the mechanical properties through thick sections.
As used herein the term "aluminium equivalent" means the total of aluminium in weight percent plus one-third of the total percentage of tin in weight percent. Thus the aluminium equivalent equals % Al+% Sn/3.